1. Field of the Invention
This invention is related to digital processing system architecture and more specifically to the protection of information stored in digital processing systems.
2. Description of the Prior Art
In the past, the development of computer programs for computers has been an extensive and time consuming task. The developers of computer programs recover the cost of such development by selling the programs. However, this is difficult to accomplish if the programs can be easily copied and used in the computer without paying the program developers.
One system of protecting programs is disclosed in U.S. Pat. No. 4,139,893 entitled "Calculator Program Security System" by Sidney W. Poland and assigned to Texas Instruments Incorporated. This patent discloses a technique that disables the downloading of a program stored in a read only memory used in a calculator. A security word or bit is located in the read only memory and if a bit in the security word is set, the calculator will not download the contents of the read only memory. Obviously, one properly skilled could circumvent this security method by using a device that does not read the security bit or react to the security bit being set and just downloads the read only memory.
Another approach to computer protection is contained in U.S. Pat. No. 4,168,396 entitled "Microprocessor for Executing Enciphered Programs" by Robert M. Best. This patent discloses a cryptographic microprocessor that includes real time enciphering and deciphering circuitry in the microprocessor itself. Another patent, U.S. Pat. No. 4,278,837 entitled "Crypto Microprocessor for Executing Enciphered Programs" also by Robert M. Best discloses variations of the cryptographic microprocessor architecture. A third patent by Robert M. Best, U.S. Pat. No. 4,319,079 entitled "Crypto Microprocessor Using Block Cipher" also discloses techniques for use in crypotgraphic microprocessors. While these techniques using enciphering do accomplish a degree of security, a special microprocessor device including hardware dedicated solely to enciphering and deciphering is required. In addition, the microprocessor device as disclosed do not include internal program memory.
Another approach is described in U.S. Pat. No. 4,120,030 by Jon Stone. In this approach, a conventional microprocessor is used except that the data memory has the data addresses transform such that the data addresses are effectively scrambled and can be unscrambled by the microprocessor in execution. While all these previously mentioned techniques provide software protection, the invention disclosed herein provides software protection for on-chip memory microcomputing devices that are interfaced to external peripheral devices.